1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. The present invention also relates to a process, a machine, manufacture, or a composition of matter. In particular, one embodiment of the present invention relates to a light-emitting element, a light-emitting device, an electronic device, a lighting device, a driving method thereof, or a manufacturing method thereof. One embodiment of the present invention further relates to an organic compound which can be used in a light-emitting element, a light-emitting device, an electronic device, and a lighting device.
2. Description of the Related Art
A light-emitting element using an organic compound as a luminous body, which has features such as thinness, lightness, high-speed response, and DC drive at low voltage, is expected to be used in a next-generation flat panel display. In particular, a display device in which light-emitting elements are arranged in a matrix is considered to have advantages in a wide viewing angle and excellent visibility over a conventional liquid crystal display device.
The light emission mechanism of a light-emitting element is said to be as follows: when a voltage is applied between a pair of electrodes with an EL layer including a luminous body provided therebetween, electrons injected from a cathode and holes injected from an anode recombine in the light emission center of the EL layer to form molecular excitons, and energy is released and light is emitted when the molecular excitons return to the ground state. Singlet excitation and triplet excitation are known as excited states, and it is thought that light emission can be achieved through either of the excited states.
In order to improve element characteristics of such a light-emitting element, improvement of an element structure, development of a material, and the like have been actively carried out (see, for example, Patent Document 1).